An automatic transmission allows a shift controller to drive a plurality of solenoid valves based on a driving speed of the vehicle, an open value of a throttle valve, and general detection conditions so as to control an oil pressure. Thus, a transmission gear of a target gear stage performs automatic shift.
An automatic transmission has an off-going element that changes from an engaged state to a released state and an on-coming element that changes from a released state to an engaged state for automatic shifting to a target shift speed. Releasing an off-going element and engaging an on-coming element is realized by controlling a hydraulic pressure supplied to respective elements.
In addition, the control of the hydraulic pressure supplied to the off-going and on-coming elements is performed by operating a control duty and controlling the solenoid valves according to the control duty.
As described above, to supply the hydraulic pressure to the automatic transmission, an oil pump is provided.
Conventionally, an oil pump system, which supplies an operating hydraulic pressure to an automatic transmission using a mechanical oil pump and an electric oil pump together, has been mainly used.
In particular, hybrid vehicles have mainly employed a control method of an oil pump system, which divides a running state of a vehicle into a stoppage section, a low-speed section, and a high-speed section, and selectively operates a mechanical oil pump and an electric oil pump.
According to the control method of the oil pump system, only the electric oil pump may be operated during the stoppage section, the mechanical oil pump and the electric oil pump may be simultaneously operated during the low-velocity section, and only the mechanical oil pump may be operated during the high-velocity section.
Recently, structures in which the mechanical oil pump, which is connected to the drive shaft of the engine, and generates power loss at all times, and causes fuel efficiency to deteriorate, is eliminated, and the electric oil pump is applied to supply oil to the automatic transmission are developed, such that revolutions per minute (RPM) of the pump is optimally controlled, thereby improving fuel efficiency.
However, when the hybrid vehicle using the electric oil pump travels in an extremely low temperature region, the hydraulic pressure discharged from the electric oil pump may do not follow a target line pressure. In this case, a motor of the electric oil pump is released, thereby a driver feels a sense of difference in the shifting.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.